Electron optical system



June 24, 1969 c. E. `RYAN ET Al. 3,452,179

ELECTRON OPTI CAL SYSTEM Filed April l2, 1967 United States Patent O U.S. Cl. 219-121 8 Claims ABSTRACT OF THE DISCLOSURE A circular filament surrounds a work piece to be bombarded and heated. A cylindrical grid adjusta'bly mounted and at high potential, surrounds the `work piece on either side of the filament. A focusing electrode at ground potential is concentrically mounted about the grid to direct electrons onto the work piece.

This invention relates generally to a system of electron optics and more particularly to a system for focusing electrons onto an insulating material whereby it is melted in a controlled manner.

Electron beam heating techniques have been applied extensively to the processing of refractory metals. One important application has been the preparation of single crystals of refractory metals by a vacuum zone melting technique in which the heat source is provided by a focused concentric electron beam impinging on the work. Excellent single crystals of tungsten, tantalum, rhenium, iridium and other refractory metals can be routinely pre- -pared by this technique.

However, the preparation of high quality single crystals of refractory insulating materials, such as sapphire, has presented many difficult problems. Insulating crystals cannot be coupled readily by induction heating techniques. Crucibles, suitable for use above 2000 C., are often subject tochemical, mechanical or metallurgical failure or to high temperature reactions that contaminate the melt. Hence, known technique of pulling crystals from the melt becomes quite limited at very high temperatures 2000 C.). The use of water cooled crucibles is restricted to materials that are sufficiently conductive for induction heating. The flame fusion technique imposes severe limitations on the chemical and structural perfection of crystals grown by this method.

The electron beam technique of this invention in its original form for zoning metals was unsuitable for zoning insulators for two reasons; first, there was no way of accelerating electrons to an insulating work piece, and second, a charge building up on the insulating material would defocus an electron beam. By inserting a positive high voltage accelerating electrode between the cathode and the work electrons from the cathode are accelerated. Many of these electrons miss the grid and give up their energy as heat when they impinge on the work. The charge which would normally build up on the insulating work is dissipated to the positive grid in a manner which is not clearly understood but may be `akin to secondary emission or thermionic emission.

Although in theory and very restricted applications electron beams could be used to refine insulators, there remained several difficulties that limited the practical preparation of high quality single crystals. First, the grid which was made of tungsten became suiciently hot so that it sagged before a zone could be -passed along a sapphire rod lwhich melts at 2050 C. This sag caused loss of control of the zone and sometimes destruction of the grid if it touched the molten zone. Secondly, with this type of 3,452,179 Patented June 24, 1969 "Ice grid it became difiicult to keep the zone narrow enough to be sustained by surface tension without loss of visibility due to closely spaced focusing electrodes. Third, there was little hope of scaling up this electron optics system to handle larger rods. Fourth, it was difiicult to adapt an afterheater to this system `so that the effects of annealing during growth on crystal imperfections could be studied.

Since the electrons are accelerated perpendicular to the equipotential lines, this invention utilizes a virtual grid in the position of the zone to be melted by using two shielded grids. With this type of arrangement, the grid temperature can be controlled by adjusting the position of the focusing or shield electrode as will be explained hereinafter. A further advantage of this invention is that the grids and focusing electrodes can be extended in the direction parallel to the work piece permitting the design of mechanically strong structures.

It is therefore lan object of this inventon to provide a new and improved electron optical system.

It is a further object of this invention to provide a new and improved electron optical system for use in the floating zone method of crystal refining.

It is another object of this invention to provide a new and improved electron optical system which may be easily adjusted.

It is still a further object of this invention to provide an electron optical system which permits the length of the heated zone to be varied.

It is still another object of this invention to provide an electron optical system for a zone refining device which permits the grids to serve as an adjustable foreheater and afterheater to provide a wide temperature range.

It is still another object of this invention to provide a new and improved electron optical system with grids that permit the heated zone to be viewed during operation.

It is still another object of this invention to provide an electron optical system 4which may be constructed :of dimensions corresponding to the size of the piece worked It is still another object of this invention to provide a new and improved electron optical system which can be used on insulators, semiconductors and metals.

It is still another object of the invention to provide an electron optical system which is economical to produce and utilizes conventional, currently available components that lend themselves to standard mass production manufacturing techniques.

These and other advantages and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiment in the accompanying drawing wherein the figure is a perspective view of the invention partly in section.

This invention would have general application in an electron beam furnace or similar heating apparatus. The electron optical system is mounted on the electron beam carriage 10 which moves the system along the length of the work piece 12 by means of the spiral shafts 13. The work piece is a poly-crystalline insulating material which is in the form of a pressed, compacted or sintered bar or rod for purposes of this disclosure, although the invention is not limited thereto. Insulating supports 14 and 16 are mounted on the carriage and may be made of boron nitride. The insulator 18 provides support for the circular iilament 20. A pair of accelerating grids 22 and 24 are threadably mounted in the insulating support. The focusing electrodes 26 and 28 are threadably mounted in the insulating supports concentrically around the accelerating grid. The threadable means of mounting the focusing electrodes Iand accelerating grids provide for adjustability of the components. The grids are hollow permitting the piece worked on to pass therethrough and further aligned in such a manner as to let the work piece pass through the center of the filament. The accelerating grid is -maintained at a high positive voltage during the operation While the focusing electrode is maintained at ground potential.

The accelerating grids and focusing electrodes are fabricated from high density carbon and the supporting insulator may be made of boron nitride. This combination of materials is particularly advantageous in that generally carbon and boron nitride are easy to machine, relatively very strong at very high temperature and can be readily purified after fabrication by outgassing the parts in a partial vacuum at a temperature of the order of 2500" C. They are also dimensionally very stable with heat treatments. Both carbon and boron nitride parts can be easily cleaned after repeated use by merely scraping or abrading any deposits from the surface with, for example, steel Wool.

In order to grow high quality crystals, it is necessary to control the growth interface at the freezing end of the molten zone. The molten zone is that area between the accelerating grids where the electrons from the filament strike the work piece. The liquid solid interface should be perpendicular to the 4axis of the rod being zoned and either straight or slightly convex towards the growing solid. The split grid configuration with both foreheater and afterheater and the correct length zone establish the conditions necessary for a llat zone interface and correct vertical and horizontal temperature gradients to produce relatively strain-free crystals. With the adjustable optics of this invention, it is possible to insure that the zone length is proper for each work piece of a different material worked upon. For example, some materials have a relatively low surface tension, hence 'when the zone is too long, the molten material tends to flow down the zone and enlarges the diameter of the lower rod. lf then, the zone is run cooler by separating the grids and focusing electrodes the liquid becomes more viscous and tends to resist rotation of the driven rod causing misalignments, the work piece may be rotated by means not shown to insure even heating and exposure to the electron beam.

. After a calibration period, it is possible with this systern to determine accurately the proper setting of the electron optics for each type material worked upon in order that perfect crystals will be forthcoming during each operation of the system. The zone length of this system is accurately determined by the accelerating grid separation.

When the material worked upon is an insulator as described herein, power may be supplied intentionally to heat the grids in Which case they will act as foi-cheaters and afterheaters which will help control the crystal interface shape.

Although the invention has been described with reference to a particular embodiment, it will be understood-to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

We claim:

1. An electron optical system for heating dielectric material comprising: a filament, a pair 0f cylindrically shaped accelerating grids mounted on opposite sides of the lament; and a pair of cylindrically shaped focusing electrodes mounted concentrically around the accelerating grid on either side of the filament, so that the material to be heated may pass through the grids and electrodes near the filament.

2. An electron optical system according to claim 1 wherein: the filament is generally circular and the material to be heated passes through the circle.

3. An electron optical system according to claim 2 wherein: the accelerating grids and focusing electrodes are adjustably mounted in an insulating material.

`4. An electron optical system according to claim 3 wherein: the insulating material is boron nitride.

5. An electron optical system according to claim 3 lwherein: the accelerating grids and focusing electrodes are carbon.

6. An electron optical system according to claim 3 wherein: the accelerating grid is at a high positive voltage.

7. An electron optical system according to claim 3 wherein: the focusing electrode is at ground potential.

8'. An electron optical system according to claim 3 wherein: said accelerating grids form foreheaters and afterheaters of the material heated.

References Cited UNITED STATES PATENTS ANTHONY `BART-IS, Primary Examiner. W. D. BROOKS, Assistant Examiner.

Us, ci. Xa. 513-82 

